1. Field of the Invention
This invention relates to a fixing device for fixing an unfixed image on carried on the surface of a recording medium onto the recording medium by heating and pressurizing the recording medium, and an image forming apparatus provided with the fixing device.
2. Description of the Related Art
As an image forming technique, an electrophotographic system has become widespread because it have many advantages that the printing speed is high, the preparation of the printing plate is not required each time, an image can be directly acquired from various items of image information, the apparatus size is relatively small, full-coloring can be easily realized, etc.
In the image forming apparatus (electrophotographic apparatus) which adopts the electrophotographic system, generally, by bringing charged toner into contact with the surface of a latent image carrier on which an electrostatic latent image is formed, toner is selectively applied to the surface of the latent image carrier to form a toner image. The toner image is transferred onto a recording medium through or not through an intermediate transfer body. The toner is fixed on the surface of the recording medium by heat and/or pressure, thereby providing an image.
In such an electrographic apparatus, generally, a fixing device in a two-roll system composed of a heating roll and a pressure roll kept in contact with each other is employed for fixing. Bypassing the recording medium, on the surface of which the unfixed toner image is formed, through a nip zone formed through contact of both rolls, the toner is molten by heat and pressure so that the toner image is fixed on the surface of the recording medium as a permanent image. As the case may be, in place of the heating roll and/or pressure roll, the heating member and/or pressure member each having an endless belt shape are employed.
The heating roll is a metallic core incorporating a heat source such as a halogen lamp, which is covered with an elastic layer or releasing layer. The surface of the heating roll is heated internally by the heat source. From the viewpoint of energy saving and preventing a user from waiting in using the image forming apparatus, it has been demanded for the fixing device that the hearing member such as the heating roll can be heated instantaneously to minimize the warm-up time.
A related art fixing device includes a fixing roll (heating roll) which is a hollow fixing body, a pressure roll which is pressed on the fixing roll by an urging unit and a halogen heater (fixing heater) serving as a heating unit. In this fixing device, electric power is supplied from a power source to the halogen heater so that the fixing roll is heated. An output from a sensor to detect the surface temperature of the fixing roll is supplied to a temperature control. The halogen heater is ON/OFF controlled on the basis of the above output by the temperature control so that the fixing roll is kept at a predetermined surface temperature. The unfixed image of the unfixed toner carried on the recording medium is heated and pressurized together with the recoding medium within the fixing nip formed by the fixing roll and pressure roll so that it is permanently fixed.
Owing to difficulty of forming a free shape of the fixing nip, the conventional fixing roll having a hollow cylindrical shape presented such inconvenience that the recording medium is wrinkled or curled, transportability is affected and limitation is given to improve the fixing property. It was difficult to obviate such inconvenience for all of various recording media by the conventional fixing roll.
For example, in the recording medium having a two or more multiple-layer structure such as an envelope, if the shape of the nip zone between the fixing roll and the pressuring roll is not planar but has a curvature, even with equal rotary speeds of both rolls, a slight difference occurs in their linear speed on the surface to give a difference in the transporting speed between the upper and lower surfaces of the recording medium. This may lead to inconvenience such as wrinkle of recording medium and affection on the capability of fixing and feeding (transportation). Particularly, when the process speed is increased, the above inconvenience is likely to occur.
In order to solve the above problem, the technique disclosed in JP-A-7-287460 has been proposed. The technique is a fixing device including a hollow pipe incorporating a heat source, a pressure roll for pressuring the hollow pipe and a heat-resistant endless sheet wound around the hollow pipe, wherein a recording sheet (recording medium) carrying unfixed toner is passed between the endless sheet and the pressure roll to implement the fixing by heating/pressurizing. In this fixing device, the surface carrying the unfixed toner of the recording sheet is in contact with the endless sheet which rotation-moves at the speed equal to the feeding or transporting speed of the recording sheet, and the pressed position of the hollow pipe by the pressure roll is practically planar.
In such a fixing device, however, the entire hollow pipe is heated by the halogen heater so that the area other than the nip zone of the hollow pipe which does not contribute to fixing is also heated. This leads to a problem that it takes a long time to increase the temperature of the entire fixing member to a predetermined temperature, thereby not shortening the warm-up time.
Where recording sheets are continuously fed, since heat is absorbed by the recording sheet, the temperature falls. In order to keep the nip zone of the hollow pipe at a predetermined temperature, heating must be carried out from the heat source. However, the area opposite to the nip zone with respect to the heat source in the hollow pipe, where heat is not absorbed, is rapidly heated to a high temperature. As a result, in order to reduce the temperature of the pertinent area to a predetermined temperature or lower, it is necessary to reduce the printing speed and stop the printing.